Typhoon Subsidence Intensity Flips Surface Ozone Spikes From Photochemical Production to Vertical Transport

Abstract Peripheral subsidence of Northwest Pacific typhoons can trigger extreme surface ozone episodes, yet the relative contributions of local chemistry and vertical transport remain poorly constrained. Here we measured 30‐min aerodynamic gradient fluxes of NO x , O 3 , and CO at 118 and 168 m on Canton Tower (Pearl River Delta) during 2 September typhoons in 2021 and 2022. Surface O 3 climbed by >50% in both events. In September 2021, moderate subsidence created hot, stagnant conditions that accelerated local photochemical ozone production, yielding upward O 3 flux. In September 2022, stronger, long‐lived subsidence instead injected ozone‐rich air from aloft, causing downward O 3 flux despite favorable chemistry. This flux evidence demonstrates that subsidence intensity toggles the balance between local production and vertical transport of ozone, informing forecasts of coastal extreme‐ozone risk.